ESR Single Crystal Study of the Mechanism of Free Radical Formation in Nitromethyl Pyrimidines

1-Methyl-5-nitrouracil, 3-methyl-5-nitrouracil, and 1,3-dimethyl-5-nitrouracil form iminoxy radicals R=N-Ȯ when exposed to ionizing radiation as do most other nitropyrimidines. The nitrogen hyperfine coupling principal values for this radical in 1-methyl-5-nitrouracil are$A_{1}=49{\rm G},A_{2}=29{\rm G}$, and$A_{3}=25{\rm G}$; the g-value principal values are$g_{u}=2.0067,g_{v}=2.0059,g_{w}=2.0020$. Additional couplings seen at a few orientations allowed identification of the complete radical structure. The mechanism postulated for nitropyrimidine radical formation involves a hydrogen addition to a nitro-group oxygen, followed by the transfer of this OH group to C(6) of the pyrimidine ring. The alkyl group on C(6) in 5-nitro-6-methyluracil and 5-nitro-6-ethyluracil prevents the second step and noniminoxy radicals are subsequently formed.